1. Field of the Invention
The present invention relates to a coolant circulation system for circulating a coolant through a coolant passage of a liquid-cooled engine, a radiator for an engine cooling system and a heat exchanger for a passenger compartment heating system.
2. Description of the Related Art
A conventional coolant circulation system in motor vehicles includes a coolant circulating circuit provided with conduits made of, e.g., flexible hoses, which extends to provide a fluidic communication between a water jacket or coolant passage formed in a vehicle engine, a radiator used in an engine cooling system and a heat exchanger used in a passenger compartment heating system. The conventional coolant circulation system also includes a coolant pump provided adjacent to and operationally connected with the engine to forcibly circulate a coolant in the circuit. The coolant flowing through the water jacket exchanges heat with the engine to cool the latter. One part of the heated coolant passing through the water jacket is directed to the radiator to thereby be cooled to an appropriate temperature, and the other part of the heated coolant is directed to the heat exchanger to warm a passenger compartment.
In the conventional coolant circulation system, the heat source for warming the passenger compartment depends only on the heat generation of the engine, and therefore, it is generally difficult to warm the passenger compartment to a satisfactory temperature until a sufficient time has passed after the engine starts, especially in cold districts. Also, it is generally difficult for the cooled engine to perform a proper combustion until the engine is self-heated to a certain degree due to the heat generation thereof, which may deteriorate a combustion efficiency and may generate a polluted exhaust gas.
U.S. Pat. No. 4,974,778 (Bertling) discloses a heating system, for a vehicle occupant space, which includes a supplementary heat source or heating unit incorporated in a coolant circuit of the heating system. In this coolant circuit, a coolant flows from a liquid-cooled engine to the heating unit to be additionally heated by the latter, and the additionally heated coolant is supplied from the heating unit to a heat exchanger of the heating system. Therefore, it is made possible to warm the passenger compartment to a satisfactory temperature even if a sufficient time has not passed after the engine starts, and even in the cold districts.
FR 2 619 206-A1 also discloses a supplementary heat source incorporated in a coolant circuit of a heating system, and teaches that the supplementary heat source can serve to warm up an engine.
In the conventional coolant circulating system, the coolant heated by the supplementary heat source is supplied to the heat exchanger arranged downstream of and adjacent to the supplementary heat source, and thereby it is possible to promptly warm the passenger compartment with the aid of the supplementary heat source. However, the coolant passed through the heat exchanger, the temperature of which has been decreased in a certain degree, is supplied to and flows in the water jacket or coolant passage of the engine, so that it is made difficult for the coolant to quickly warm up the engine. Therefore, it is difficult to run a motor vehicle in a proper condition of, e.g., the combustion efficiency of the engine, immediately after the engine starts or until the engine is sufficiently self-heated. Also, the heat generated from the engine, which is generally most useful in the heating system for the passenger compartment, is not fully utilized in the heating system.